This invention relates to the field of automatic testing of electronic circuit boards, and, more particularly, relates to test fixtures used in automatic test systems.
In recent years, automatic test systems have been developed for the testing of circuit boards prior to their being installed in ultimate products. For example, a manufacturer of television sets might utilize an automatic test system to verify that all the circuit boards for a set are good before installing them in the set. Some testing systems have the capability of performing both in-circuit tests and functional tests. In-circuit tests involve the testing of the discrete components on a circuit board by electrically isolating each component and subjecting it to known test voltages and currents and measuring its output. The electrical isolation of components requires the use of numerous electrical probes which contact the circuit under test at predetermined nodal points. This can involve the use of 1,000 or more probes depending upon the circuit under test. A functional test involves the introduction of a test signal on the input leads of a circuit board and measurement of the response on its output leads to determine whether the board functions overall in the intended manner. Again, the contact with the input leads and output leads is typically made by means of electrical probes which, in turn, are connected to the test system.
It is, of course, well known to those skilled in the art that the presence of a great number of probes in electrical contact with the circuit under test produces certain undesirable inductance and capcitance effects in the circuit. Naturally, the test results are distorted to some extent and the degree of distortion must be taken into account in interpreting the test results. These unwanted inductance and capacitance factors are particularly undesirable in functional tests. Therefore, it would be desirable if only those probes necessary for a functional test would be brought into contact with the circuit board during such tests. Various attempts have been made to provide a test fixture which would have the capability of advancing only selected probes for different parts of an automatic test program. For example, U.S. Pat. No. 3,714,572 discloses a fixture in which the individual probes are pneumatically advanced into contact with the circuit under test. While test fixtures of this type would appear to eliminate unwanted inductance and capacitance effects, there are certain disadvantages involved. For one thing, as the number of probes needed becomes large, such test fixtures become unduly expensive. Secondly, because of the minimum practical size of the cylinders involved, there is a limitation as to how close one probe can be placed to another. The presence of a large number of cylinders adds unnecessarily to the bulk of these test fixtures. The need for valving and conduits for such individual probe, as well as cam-operated controls, introduces a significant level of complexity in test fixtures of this kind. With this type of test fixture, it is necessary to provide clamps or other devices which will hold the circuit board under test in place as the probes are advanced into contact with it during the test.
As is well known to those skilled in the art, another type of test fixture utilizes a vacuum to hold the circuit board in place during the test and to cause the test probes to advance into contact with the board. With fixtures of this kind, the circuit boards are placed on and removed from the test fixture more readily since there are no clamps or other devices to be manipulated. This tends to reduce the cycle time needed to test each board, thereby increasing productivity. As noted above, some modern test systems are capable of conducting both functional and in-circuit tests. It would, therefore, be desirable to provide a vacuum operated test fixture which would have the capability of advancing at one time only those probes necessary to conduct a functional test of the board and to advance all probes at another time to conduct the in-circuit tests. It would be particularly desirable if a test fixture of this type could be produced at a relatively economical cost through the introdcution of a few new parts to existing vacuum-operated fixtures.